A 22-year-old woman presented with a palpable left thigh mass that
had recently become tender. Her medical history was otherwise
unremarkable.

IMAGING FINDINGS

Ultrasound images revealed a 7 x 5 x 3-cm vascular mass lying
adjacent but not fixed to the anterior femur. Magnetic resonance imaging
(MRI) of the left thigh with and without contrast was performed. MRI
revealed a 6.5 x 6.0 x 4.5-cm mass. The mass originated 12.5 cm above
the knee joint situated between the anterior medial left femur and
vastus medialis muscle. There was smooth displacement of the vastus
medialis muscle without obvious invasion of either the femur or
musculature. This mass was isointense to muscle on T1-weighted (T1W)
images (Figure 1), was heterogeneously bright on T2-weighted (T2W)
images (Figure 2), and enhanced rather homogenously on postcontrast
images (Figure 3). There was no evidence of hemorrhage or mass. Flow
voids were appreciated within the proximal portion. At least 3 enlarged
vessels acting as a vascular pedicle for the tumor were seen on the
source images. A left-lower-extremity arteriogram (Figure 4) revealed a
hypervascular tumor that received the majority of its blood supply from
the profunda femoral artery.

During the arteriogram, the neoplastic vessels were embolized using
polyvinyl alcohol, Embosphere Microspheres (BioSphere Medical, Rockland,
MA), and microcoils. Surgical excision of the mass was then performed.
The patient subsequently underwent a whole-body bone scintigraphy
(Figure 5) and computed tomography (CT) scan of the chest (Figure 6).
Both showed probable metastatic disease.

[FIGURE 1 OMITTED]

DIAGNOSIS

Alveolar soft part sarcoma (ASPS)

DISCUSSION

Alveolar soft part sarcoma was first described in 1952 by
Christopherson, Foote, and Stewart as a soft tissue sarcoma with unique
clinical and pathological features. (1) The name was derived from its
microscopic appearance. Alveolar soft part sarcoma is a high-grade
malignant tumor comprising 0.5% to 1% of all soft tissue sarcomas in
adults and 0.8% to 1.8% of those in children. (2,3)

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Alveolar soft part sarcoma most commonly presents between the ages
of 10 and 35 years, with women being diagnosed on the average at 20 to
22 years and men at 27 to 30 years. It has been documented in children
as young as 2 years of age. (4) Alveolar soft part sarcoma is overall
more common in females; however, after the age of 30, men are more
frequently diagnosed. The most common site of origin is within the lower
extremity, particularly the fascial planes or skeletal muscle of the
anterior thigh. In children, the head and neck region is a more common
site of origin, with the orbit and tongue as predominant locations.1 The
usual presentation is that of a slow-growing, painless mass within the
lower extremity without other symptoms. Possibly because of this
insidious onset, the tumor size is usually quite large and metastasis is
detected in approximately 20% to 33% of patients at the time of initial
diagnosis. (2,4) The most common locations of metastasis are lungs,
bone, and brain, in that order. Therefore, whole-body bone scintigraphy
and chest CT are required for staging. Bones may be affected by direct
or metastatic spread; however, distant bony metastasis is much more
common, and local bony extension is reported as relatively rare. (5)
Many cases of metastasis may be detected as late as 10 years after the
initial diagnosis.

[FIGURE 5 OMITTED]

Macroscopically, the tumor is well circumscribed and usually
measures 3 to 6 cm. Tumors up to 20 cm have been reported. (6) The
larger tumors often exhibit hemorrhage and necrosis. Alveolar soft part
sarcoma has a characteristic microscopic appearance. Nests of large
granular cells surrounded by capillaries are seen in a pattern
resembling alveoli. These cells contain periodic acid-Schiff
(PAS)-positive and diastase-resistant crystals, which helps make the
diagnosis a certainty. (2) The histogenesis of ASPS is still unclear.
Muscle-associated proteins are often found in immunohistochemical
studies, suggesting a myogenic origin; however, there is conflicting
data regarding these studies. This remains an area of continued
research.

[FIGURE 6 OMITTED]

Radiographic findings, though not specific, can, when combined with
an adequate history, usually lead to the correct diagnosis. Ultrasound
will typically show a hypoechoic, hypervascular mass. CT may reveal a
well-circumscribed mass with marked contrast enhancement. MRI is the
most sensitive and specific imaging modality. Typical findings include
isointensity to a slight increase in intensity on T1W images, increased
signal with T2W sequences, and strong enhancement. Multiple large flow
voids consistent with peritumoral vessels may also be appreciated. (2)
Angiography shows a highly vascular lesion with enlarged feeding
arteries, arteriovenous shunts, and delayed washout. As mentioned
previously, a CT chest and whole-body bone scan are performed for
staging. These studies will show a typical appearance for metastatic
disease if present.

Despite its slow growth, ASPS has a grave prognosis. Several
studies report a 5-year survival rate of approximately 60% with a range
of 45% to 88% depending on the selection of the patients. Two of these
studies reported a median survival time of only 6 to 7 years. (2,6) The
size of the tumor and the presence of metastasis have been implicated as
the main prognostic factors. There is some evidence to suggest that age
is also a significant prognostic factor, with children exhibiting a much
longer survival time than those diagnosed at an older age. (7) The
initial treatment of ASPS is radical excision with negative surgical
margins. Local recurrence is uncommon if complete excision is performed.
The metastatic disease is the most difficult aspect of treatment. If it
is possible, the resection of pulmonary metastatic lesions can
significantly increase survival time. Chemotherapy and radiation have
shown inconsistent results and may be used successfully with individual
patients.

CONCLUSION

Alveolar soft part sarcoma is a rare soft tissue sarcoma that
presents most commonly within the lower extremity fascial planes or
skeletal muscle of young females. Lung, bone, and brain metastasis are
common. Median survival time is 6 to 7 years, depending upon the age of
the patient, the size of the tumor, and the extent of metastasis.
Imaging is nonspecific. Isointensity on T1W images, hyperintensity on
T2W images, enhancement, and multiple flow voids are common MRI
findings.